It's possible, but why would you want to? Most people go the other way!

You already have at least Exchange and a DC. To go to a SBS, you would still need a new box. So, why not just put Exchange on the new box, and save a lot of time and hassle?

If you really like the road less traveled, here's some reading material for you:http://support.microsoft.com/default.aspx/kb/884453
As you can see, the process is quite involved, and once you start, you have a time limit where you have to have all steps completed within a certain timeframe.